Aramid paper is synthetic paper composed of aromatic polyamides. Because of its heat resistance, electrical insulating properties, and flexibility, said paper has been used as electrical insulation paper. Of these materials, Nomex.RTM. of Du Pont (U.S.A.) is manufactured by mixing poly(metaphenylene isophthalamide) flocks and fibrids and then subjecting the mixture to hot-press calendering, and is known as electrical insulation paper with excellent electrical insulation papers and with strength which remains high even at high temperatures.
Not only has Nomex.RTM. paper, due to its resistance to radiation, been used in electrical equipment designed for nuclear power plants and highly radioactive environments, but its use in high-temperature labels, in which the heat resistance of this type of paper can be utilized, has also been the subject of investigations in recent years. [Such paper,] however, contains highly crystalline poly(metaphenylene isophthalamide) flocks (as has been described above), and these flocks hardly change their shape even when subjected to hot-press treatment. The result is not only that it is impossible to ensure high smoothness of the sheet surface, but also that fuzzing of the surface flocks inevitably occurs, and this impairs the printing properties [of the paper] and limits its applicability as labelling paper. In particular, the paper must have a higher level of surface smoothness than does conventional aramid paper when bar codes are to be printed on such high-temperature labels.
In addition, high-voltage (500,000 V) power transmission has recently been utilized, and the utilization of ultrahigh-voltage (1,000,000 V or higher) power transmission (UHV power transmission) is expected in the near future. Electrical insulation paper to be used in transformers for such high-voltage and ultrahigh-voltage power transmission must have high resistance to fuzzing because even microscopic fuzz fibers (about 50 .mu.m) may cause a local insulation puncture.
Japanese Laid-Open Patent Application 4-6708 discloses an electrical insulation paper whose fuzzing is reduced by the hot pressing of a two-layer structure obtained by laminating synthetic paper; said paper is composed of 100% of poly(metaphenylene isophthalamide) pulp and weighs 10 g/m.sup.2 or more. The application also describes synthetic paper which is composed of poly(metaphenylene isophthalamide) pulp and poly(metaphenylene isophthalamide) staple fibers.
However, poly(metaphenylene isophthalamide) fibrids retain water extremely well and drain water poorly, so an increase in the weight of 100% fibrid paper impairs its paper-making properties. In addition, because the synthetic paper thus manufactured is characterized by low tear strength and appreciable thermal contraction, an increase in the proportion of this paper during its lamination and manufacture into electrical insulation paper lowers the tear strength, renders thermal contraction more pronounced, produces sheets which curl readily, and causes other practical problems, which become more evident with a decrease in the weight of the aramid paper as a whole.
When only one side of laminated paper is composed of the above-mentioned 100% fibrid layer, the danger that fuzzing will occur on the other side still remains, limiting the scope of applications. When attempts are made to overcome this drawback by using the above-mentioned 100% fibrid layers on both sides, it becomes difficult to control not only the drawbacks described above, but also the treatment conditions maintained during hot rolling. Specifically, a 100% fibrid layer such as the one described above is less permeable to gases than the above-mentioned layer composed of a mixture of flocks and fibrids, so when hot wiling is performed after 100% fibrid layers have been used on both sides, it is difficult to close the pores inside the sheet, and processing can easily go wrong.
Therefore, whether 100% fibrid layers are located on one side or on both sides, the weight should still be as low as possible.
Because of the reasons described above, conventional aramid paper had surface smoothness which was too low for the paper to be suitable for bar code printing. For example, the Oken-type smoothness of a 2-mil product is 22 sec per 10 cc and gradually diminishes to 14, 7, 4, and 3.5 sec per 10 cc as the thickness of the product increases to 3, 5, 7, and 10 mil. Of the types of paper used for bar code printing, the minimum smoothness of a high-quality paper is 40 to 70 sec per 10 cc. When a bar code is printed on the above-mentioned aramid paper using a thermal transfer printer, many lines are broken even on a 2-mil product, which has the best smoothness, so it is impossible to obtain the same printing quality as with high-quality paper, and products three or more rail in thickness do not withstand use at all. Therefore, a smoothness which is at least comparable to that of the high-quality paper (i.e., 40 sec per 10 cc or higher) is needed for paper to be suitable for printing bar code labels. The smoothness should be 70 sec per 10 cc or higher, and preferably 100 sec: per 10 cc or higher.